1. Field of the Invention
The present invention relates to a metal vapor discharge lamp which is operated by applying high voltage generated in an actuation of a heat sensitive switch means.
2. Description of the Prior Arts
Recently, a high pressure sodium vapor discharge lamp and a metal halide discharge lamp have been remarkably developed to replace the conventional high pressure mercury vapor discharge lamp. The energy saving of such discharge lamps has been highly evaluated. Thus, such discharge lamps have higher starting voltage than that of the high pressure mercury vapor discharge lamp and can not be operated by a ballast for the high pressure mercury vapor discharge lamp and has been operated by a special ballast which is equipped with a starter for generating high voltage pulse such as several thousands volts. Such special ballast has larger size, heavier weight and higher cost than those of the ballast for the high pressure mercury vapor discharge lamp. These disadvantages are trouble for higher commercial expansion of the high pressure sodium vapor discharge lamp or the metal halide discharge lamp.
In order to prevent such troubles and to attain the rapid commercial expansion of such discharge lamps, various improvements have been proposed.
Among these proposals, the optional proposal in the practical application, is to operate the metal vapor discharge lamp such as the high pressure sodium vapor lamp and the metal halide lamp by using the conventional ballast for the high pressure mercury vapor discharge lamp. In order to operate the metal vapor discharge lamp, it requires a pulse voltage having several thousands volts for the initiation. In order to generate such pulse voltage, the circuits shown in FIGS. 1 and 2 have been used. In the conventional method, the heat sensitive switch means (4) such as a bimetallic switch is placed in an outer bulb (7) of the discharge lamp. The lamp is operated by utilizing the high pulse voltage (surge voltage) generated in the ballast (8) at a moment detaching the contact point by the operation of the heat sensitive switch means (4). It is possible to operate the metal vapor discharge lamp by a compact, light weight, economical ballast for the high pressure mercury vapor discharge lamp by connecting the heat sensitive switch means (4) in the outer bulb (7) without placing a starter in the ballast (8). However, in the conventional method placing the heat sensitive switch means (4) in the outer bulb (7), it is possible to operate the metal vapor discharge lamp such as the high pressure sodium vapor discharge lamp by the ballast for the high pressure mercury vapor discharge lamp. When the arc tube (1) is in the normal condition, the discharge lamp can be operated without failure by surge voltage given by operation of the heat sensitive switch means (4). There is no trouble in the practical application. When the arc tube (1) is not operated in the normal condition, or is not easily initiated, or it is in the last term of the life, the surge voltage at the operation of the heat sensitive switch means is 10 to 20 [KV], whereby the insulation breakdown of the ballast (8) and the operating circuit may be caused. The breakdown of parts of the ballast or the operating circuit is frequently caused to be inoperative.
In order to decrease the surge voltage and to decrease the insulation breakdown, it has been proposed to have the structure shown in FIG. 2 wherein the current limit resistor (6) is connected in series to the heat sensitive switch means (4) so as to decrease the current passing through the contact point of the heat sensitive switch means (4). In such method, the surge voltage of about 10 [KV] is generated in the operation of the heat sensitive switch means in the abnormal condition of the arc tube (1), whereby the insulation breakdown of the ballast and the operating circuit may be caused.